1. Field of the Invention
This invention relates to a local oscillating circuit for use in a satellite-broadcast receiving converter or a measuring equipment, etc.
2. Related Background Art
In association with the recent rapid development of information network systems, a satellite communication and broadcasting system has been required, and shifting of the frequency band of the system to a higher band also has been required. In order to satisfy these requirements, various circuit elements such as a field effect transistor, etc. which can be operated in a high frequency band have been studied and developed. A Schottky-barrier type electric field effect transistor (MESFET) formed of a compound semiconductor has been used such as a field effect transistor for a high frequency band. Especially a GaAs compound semiconductor transistor has been employed because it can break-through a limitation in characteristics of an Si bipolar transistor. In addition, in order to satisfy various requirements such as miniaturization, low-cost and high performance for a system, integration of a first-stage amplifying portion of a down-converter has been recently constructed as a MMIC(Microwave Monolithic Integrated Circuit).
An ordinary GaAs-MESFET formed by ion-implantation has been conventionally used in a local oscillating circuit which is used in a satellite-broadcast receiving converter and so on. A conventional oscillating circuit using the ordinary GaAs-MESFET as described above inherently has an excessively large phase-noise without modification, and this phase-noise is caused by "low Q" which is inherent to the GaAs-MESFET. There are various causes to induce the phase-noise of the oscillating circuit, and a low-frequency noise of an FET (considered as one of 1/f noise because the spectrum is inversely proportional to frequency) and a bias-dependence of a gate capacitance Cgs have been known as main causes.
Therefore, in order to implement an oscillating circuit having a low phase-noise, a dielectric resonator having high dielectric constant has been hitherto provided at the outside of the oscillating circuit to obtain a desired resonance spectrum.
As described above, the conventional oscillating circuit using the GaAs-MESFET is required to be added with the dielectric resonator to reduce the phase-noise at least when it is used in the satellite-broadcast receiving converter. This requirement is also made for an MMIC oscillating circuit in which all elements inherently requisite for the circuit construction of the oscillating circuit are integrated on one chip.
FIGS. 1A and 1B show an example of the conventional MMIC oscillating circuit, in which FIG. 1A is a circuit diagram, and FIG. 1B is a substantially plane view of the arrangement of the circuit. In the MMIC oscillating circuit as shown in FIGS. 1A and 1B, an FET 11, resistors 12a and 12b, and capacitors 13a to 13d are formed on an MMIC substrate 1, and the MMIC substrate 1 having the above elements is mounted on a ceramic substrate 3 on which a microstrip line 2 and other circuit patterns have been formed. The microstrip line on the ceramic substrate 3 and the MMIC substrate 1 are connected to each other by a wiring 4, and a dielectric resonator 5 is mounted adjacently to the microstrip line 2 so as to be electromagnetically coupled to the microstrip line 2. In addition, an end terminal resistance 6 is provided on the ceramic substrate 3.
As described above, since the conventional MMIC oscillating circuit has been so designed that the dielectric resonator and the microstrip line electromagnetically coupled to the dielectric resonator are provided at the outside of the MMIC substrate, an effect of utilization of an MMIC to promote the miniaturization of the system can not be sufficiently obtained.